Visualizing modes

The Graves on SOHO VoIP blog tipped us off to this cool video showing the vibration of a square plate at different frequencies. By covering the plate with salt, we can see the areas where the plate vibrates a lot (the salt rolls away) and the areas where it doesn’t vibrate at all (the salt collects). These spots and lines of little or no movement are called nodes. As the driving frequency (and sound) gets higher and higher, the patterns (called mode shapes) get more and more complex (and cool looking!)

This behavior is a great example of why we don’t use big speakers (woofers) to generate high-frequency (treble) sound. Instead of moving in unison like a piston, the speaker cone resonates internally at high frequencies, and different parts of the cone are moving in different ways, like a group of uncoordinated smaller speakers. This causes a poor frequency response, with lots of peaks and dips — often referred to as “breakup”. Smaller drivers (tweeters) are more at home with high-frequency sound, since their resonant “breakup” occurs at frequencies near or beyond the limit of hearing.

This is also a great way to visualize the analogous effect of room modes. Instead of a vibrating plate, a room is full of vibrating air, and at certain frequencies there will be points or areas in a room’s volume — nodes — with very little sound. This is most pronounced in hard, reverberant rooms, and at low frequencies. Still, even though the effect is more subtle in normally absorptive rooms, it can wreak havoc with the reproduction and recording of low-frequency sound, so identifying and managing room modes is a common task in the design of recording studios and listening rooms.

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